Field of the Invention
The invention relates to a method and an apparatus for adapting a transmission bit rate of each individual one of a plurality of connections to an available bit rate capacity of a data multiplexer operating according to an asynchronous transfer mode.
An asynchronous transfer mode (ATM) of a standard which is defined by the CCITT (Comitte Consultatif International Telegraphique et Telephonique) and is described in general in an article entitled: "ATM-High Speed mit drei Buchstaben" ATM: High Speed in Three Letters!, Funkschau 7-1995, pp. 40-43, is a switching method in which information is bundled in cells, and the cells are exchanged not necessarily periodically between a sender and a receiver. In switching the cells, the data of the sender are combined into portions of uniform size, such as so-called packets or cells, which are then each provided with a top line known as a header. The header includes information on the sender and the receiver. The network, for instance, includes interlinked computers as data multiplexers. Each of them reads the headers of the arriving packets and decides accordingly where they should be sent next, so that they come closer to the destination. The data multiplexers can also ascertain whether or not a connecting line has failed, so that they can then choose another route for the data transmission without bothering the sending or the destination computer about it. Under some circumstances, in some systems, the packets are conveyed over different routes and as a result do not arrive in the correct order. In that case, the destination computer must reassemble the packets correctly. A mixup in the arriving packets can also occur if one had to be re-sent because a transmission error had been ascertained. In so-called virtual data packet switching, not only the destination computer but also the path and/or possible detour paths are included in the header of the data packet, so that all of the packets reach their destination over the same route.
In data packet switching, the sender can transmit at high speed while the addressee only receives at a lower speed. The data transmission is thus not interrupted but merely throttled, since excess data packets can be buffer-stored in the data multiplexers. However, data traffic jams can occur if the packet flow exceeds the storage and transmission capacity of the network. Then the various data multiplexers must simply discard packets that they can no longer store in memory or pass on. Methods for discovering and preventing network overloads are therefore important for the further development of data packet switching. In certain services, a minimum data transmission rate must also not be undershot, an example being a video conference, in which a minimum data flow is necessary to prevent the video and audio information from being "chopped up".
In a particular traffic type of asynchronous transfer mode, namely available bit rate (ABR) traffic, the bit rate of the data senders is controlled by the network. In the case of ABR connections, the network continuously ascertains the optimal bit rate for each participant, in order to achieve the maximum possible utilization of the capacity of the connections, and it informs the participants of this in the form of explicit bit rates. The ABR method is described, for instance, in the publication entitled: "The Enhanced Rate Based ABR Scheme" BRAVE-WP1.3-ROK-95001-TD-CC/b, March 1995, and is especially suitable for applications in which the participants are represented by personal computers or servers. Those devices engender stop and go traffic. In other words, phases of maximum bit rate and resting phases can alternate with one another. Both the inactive and the active phases fluctuate quite markedly over time. The phase duration may be in the millisecond range, or in the minute or even hour range, depending on the application. Since both bit rates up to the maximum possible bit rate and pauses, and all of the values in between can occur unpredictably, it is very difficult for the network to multiplex various data senders through one connection using statistical methods. The principle on which the ABR method is based assumes that a plurality of connections together use a given transmission capacity. The transmission capacity is divided "fairly" between the active data senders. The network and the data multiplexer or multiplexers in it in particular have the task of forming a distribution value (known as a fair share) that represents the division among the various data senders, and of informing the data senders of it.
In order to ascertain the fair share, in the simplest case the total transmission capacity available is divided by the number of active connections. As a rule, however, such a distribution is not expedient, since connections can also occur that may be limited to a lesser bit rate than the one allocated to them. The result in that case would actually be non-optimal utilization of the available transmission capacity. A new fair share value is therefore ascertained, which divides the transmission capacity not used by one connection among the other active connections. If another connection is then added, an overload situation quickly arises, since the bit rates of all of the connections together exceed the available transmission capacity. The overload situation persists until the fair share value is suitably adapted. Meanwhile, the excess cells are buffer-stored in the data multiplexer. However, the overload situation may also occur if the limitation to a certain bit rate is, for instance, eliminated. In that case again, adaptation of the fair share value is necessary.
In order to inform the participants of the explicit transmission rate, that is the desired bit weight, special ATM cells, known as capacity management cells (or RM cells, where RM stands for Resource Management) are used. Each participant periodically arranges its cells in a so-called user cell stream, specifically either after a certain period of time or after a certain number of user cells. The receiving participant loops the RM cells back and inverts the directional bit, in order to identify those cells as reverse-flowing RM cells. The data multiplexers update the explicit bit rate field of the returning RM cells only in the event that their current fair share is smaller than the bit rate indicated by the RM cells. Thus the connecting element with the least available capacity determines the transmission rate for the particular connection.
It is desirable for the optimal fair share value to be determined as accurately as possible and automatically in the shortest possible time, and to assign the current fair share automatically to the RM cells moving past.